1. Field of the Invention
The present invention relates to an optical element suitable for a projection display apparatus, and a method for fabricating the optical element.
2. Description of the Related Art
Japanese Patent Laid-open Gazette No. 7-294906 discloses an optical element, called polarization converting element, for use in converting light having random polarization directions to light having one polarization direction. Such an optical element is shown in plan view in FIG. 1(A) and in perspective view in FIG. 1(B). This optical element comprises a polarization beam splitter array 22 comprising alternately adhered linear polarization beam splitters 30 having polarization splitting films 36 and linear prisms 40 having reflecting films 46. Portions of the exit surface of the polarization beam splitter array 22 are selectively provided with xcex/2 optical phase plates 24.
The linear polarization beam splitter 30 includes two rectangular prisms 32, 34 and the polarization splitting film 36 formed at the slant plane constituted by the interface between the rectangular prisms 32, 34. During fabrication of the polarization beam splitter 30, the polarization splitting film 36 is formed on the slant plane of one of the rectangular prisms and the two rectangular prisms 32, 34 are then bonded with an optical adhesive.
The linear prism 40 includes two rectangular prisms 42, 44 and the reflecting film 46 formed at the slant plane at the interface between rectangular prisms 42, 44. During fabrication of the prism 40, the reflecting film 46 is formed on the slant plane of one of the rectangular prisms, and the two rectangular prisms 42, 44 are then bonded with an optical adhesive. The reflecting film 46 is formed of an aluminum or other metal film.
Multiple linear polarization beam splitters 30 and linear prisms 40 prepared in this manner are adhered alternately with an optical adhesive to fabricate the polarization beam splitter array 22. The xcex/2 optical phase plates 24 are then selectively bonded to the exit surface of the linear polarization beam splitter 30.
Light including an S polarized light component and a P polarized light component enters from the incident surface. The incident light is first separated into S polarized light and P polarized light by the polarization splitting film 36. The S polarized light is reflected at substantially a right angle by the polarization splitting film 36, is further reflected at a right angle by the reflecting film 46, and exits the prism 40. The P polarized light passes straight through the polarization splitting film 36, is converted to S polarized light by the xcex/2 optical phase plate 24, and exits therefrom. As a result, a light beam having random polarization directions entering this optical element emerges entirely as an S polarized light beam.
The conventional optical element shown in FIGS. 1(A) and 1(B) has four rectangular prisms 32, 34, 42, 44 adhered by optical adhesive. Between entering and exiting the optical element, the S polarized light and P polarized light must therefore pass repeatedly through the optical adhesive layers formed at the prism interfaces. Since the optical adhesive absorbs some of the light, the intensity of the light decreases with each passage through an optical adhesive layer. This results in a considerable decline in light utilization efficiency.
Accordingly, an object of the present invention is to enhance the light utilization efficiency of the optical element.
Another object of the present invention is to provide the optical element which is easy to fabricate.
In order to attain at least part of the above and other objects, the present invention provides an optical element comprising a plurality of first transparent members and a plurality of second transparent members, which are alternately arranged with and secured to each other. Each of the plurality of first transparent members has a first incident surface and a first exit surface substantially parallel to each other, first and second film forming surfaces substantially parallel to each other and making a prescribed angle with the first incident surface and the first exit surface. A polarization splitting film is formed on the first film forming surface, and a reflecting film is formed on the second film forming surface. Each of the plurality of second transparent members has a second incident surface and a second exit surface parallel to each other. The plurality of second transparent members are alternately arranged with and secured to the plurality of first transparent members at the first and second film forming surfaces across the polarization splitting film and the reflecting film respectively so that the second incident surfaces are aligned with the first incident surfaces to form an incident plane and that the second exit surfaces are aligned with the first exit surfaces to form an exit plane.
In the above optical element, after the light enters through the incident surface of the first transparent member, the polarized light component thereof reflected by the polarizing-splitting film is reflected by the reflecting film without passing through a layer of optical adhesive and then exits from the optical element. The light utilization efficiency is improved because the number of times this polarized light component passes through layers of the optical adhesive can therefore be reduced.
In a preferred embodiment, the reflecting film has a dielectric multi-layer film. A reflecting film formed of a multi-layer dielectric films enables the reflectance for a specific linearly polarized light component to be increased over that in the case of a reflecting film formed of an aluminum or other metal film. A further increase in the light utilization efficiency can therefore be attained.
In the embodiment, the optical element further comprises polarization direction converting means associated with either of the first exit surface and the second exit surface. Linearly polarized light components of different polarization direction exit from the exit surface portion of the first transparent member and the exit surface portion of the second transparent member. Thus, by providing a polarization direction converting means on one of the exit surface portions, the light beam exiting from the optical element can be entirely converted to one linearly polarized light component.
The optical element may further comprise light shielding means associated with the second incident surface. If light enters from the second incident surface of the second transparent member, this light will, after reflection by the reflecting film, pass through optical adhesive layers repeatedly before being converted into S polarized light and P polarized light by the polarization splitting film. If this kind of light is shut out by providing light shielding means with respect to the second incident surface of the second transparent member, repeated passage of the light entering the optical element through optical adhesive layers can be prevented.
The optical element further comprises adhesive layers between the first and second transparent members, and at least one of a thickness of the adhesive layers and thicknesses of the first and second transparent members are adjusted to make intervals between the polarization splitting films and the reflecting films substantially constant throughout the optical element. Since this makes the intervals between the polarization splitting films and the reflecting films equal, the positional accuracy of the films in the optical element can be improved to increase the light utilization efficiency.
Preferably, the thickness of the second transparent members is set smaller than the thickness of the first transparent member. More preferably, the thickness of the second transparent member is in the range of 80% to 90% of the thickness of the first transparent member. For example, the thickness of the first transparent members is equal to a value obtained by adding twice the thickness of the adhesive layers to the thickness of the second transparent members.
The optical element may be used with a plurality of small lenses which will be arranged on the incident plane, and the intervals between the plurality of polarization splitting films may substantially correspond to a pitch of the plurality of small lenses. This makes the intervals between the polarization splitting films and the reflecting films constant, thereby increasing the light utilization efficiency of the optical element.
In another embodiment, at least one of a thickness of the adhesive layers and thicknesses of the first and second transparent members are adjusted to make the intervals between the plurality of polarization splitting films substantially correspond to a pitch of an optical axes of the plurality of small lenses. Since this provides a configuration enabling each of multiple light beams exiting from multiple small lenses to fall incident on a polarization splitting film associated therewith, it improves the light utilization efficiency.
In a further embodiment, the plurality of small lenses have a plurality of different optical axis pitches, and at least one of the thickness of the adhesive layers and the thicknesses of the first and second transparent members are adjusted to make the intervals between the plurality of polarization splitting films substantially correspond to the plurality of different optical axis pitches. This provides a configuration which, even when the pitch of the lens optical axes varies, enables each of the beams exiting from the small lenses to fall incident on a polarization splitting film associated therewith. It therefore improves the light utilization efficiency.
The optical element can be used with a plurality of small lenses which will be arranged on the incident plane. In this case, intervals between the plurality of polarization splitting films may substantially correspond to a pitch of a plurality of light beams exiting from the plurality of small lenses. The pitch of the light beams exiting from the small lenses does not always coincide with the pitch of the lens optical axes. This configuration enables each light beam exiting from the small lenses to fall incident on the associated polarization splitting film even in such a case. It thus improves the light utilization efficiency.
At least one of the thickness of the adhesive layers and the thicknesses of the first and second transparent members may be adjusted to make the intervals between the plurality of polarization splitting films substantially correspond to the pitch of the plurality of light beams exiting from the plurality of small lenses.
According to an aspect of the present invention, there is provided a method for fabricating an optical element. The method comprises the steps of: (a) providing a plurality of first transparent members each having substantially parallel first and second surfaces, and a plurality of second transparent members each having two substantially parallel surfaces; (b) forming a polarization splitting film on the first surface of each the first transparent member; (c) forming a reflecting film on the second surface of each the first transparent member; (d) alternately arranging the plurality of first transparent members each having the polarization splitting film and the reflecting film and the plurality of the second transparent members, and adhering the plurality of first transparent members to the plurality of the second transparent members; and (e) cutting the alternately adhered transparent members at a prescribed angle to the first and second surfaces to produce an optical element block having an incident plane and an exit plane which are substantially parallel to each other.
The method may further comprises the step of (f) polishing the incident plane and the exit plane of the optical element block.
In a preferred embodiment, the step (d) comprises the steps of: alternately stacking the plurality of first transparent members and the plurality of second transparent members with layers of photo-curing adhesive therebetween; and adhering the stacked first and second transparent members through exposure of light. Since this enables the optical adhesive to be cured by irradiating the adhered transparent members with light, it facilitates the fabrication of the optical element.
The step (d) may comprise the steps of: (1) forming a stack by stacking one of the plurality of first transparent members and one of the plurality of second transparent members with a layer of photo-curing adhesive therebetween; (2) curing the photo-curing adhesive layer by irradiating the stack with light; and (3) alternately stacking one of the plurality of first transparent members and one of the plurality of second transparent members on the stack with layers of the photo-curing adhesive therebetween, respectively, while curing the individual photo-curing adhesive layers by irradiating the stack with light each time one transparent member is added. Since this enables the adhesive to be cured after each transparent member is stacked, it makes it possible to establish the positional relationship among the transparent members with good accuracy.
In another embodiment, the step (d) comprises the steps of: (1) forming a stack by stacking one of the plurality of first transparent members and one of the plurality of second transparent members with a layer of photo-curing adhesive therebetween, (2) curing the photo-curing adhesive layer by irradiating the stack with light to produce a unit stack, and (3) stacking a plurality of unit stacks obtained by the steps (1) and (2) with layers of the photo-curing adhesive therebetween, respectively, while curing the individual photo-curing adhesive layers by irradiating a stack of the unit stacks with light each time one unit stack is added. Since this method also enables the adhesive to be cured after each transparent member is stacked, it makes it possible to establish the positional relationship between adjacent transparent member members with good accuracy.
Preferably, the irradiation is conducted in a direction not parallel to the surfaces of the transparent members. Since this enables the adhesive to be efficiently irradiated by the light, it reduces the adhesive curing time and improves the optical element production throughput.
According to another aspect, the present invention provides an projection display apparatus comprising the above stated optical element; polarization converting means for converting light exiting from the optical element to one type of polarized light; modulating means for modulating the light exiting the polarization converting means as a function of a given image signal; and a projection optical system for projecting the light modulated by the modulating means on a screen. The use of the optical element with high light utilization efficiency ensures projection of a bright image on the projections surface.
According to still another aspect of the present invention, an optical element comprises: a plurality of polarization splitting members, each comprising: a light incident surface; a light exit surface substantially parallel to the light incident surface; a polarization splitting film formed at a prescribed angle with the light incident surface and the light exit surface; and a reflecting film substantially parallel to the polarization splitting film. The plurality of polarization splitting members are arranged in a form of a matrix, and the polarization splitting film and the reflecting film are a dielectric multi-layer film. The light for curing the photo-curing adhesive passes through the dielectric multi-layer film. Accordingly, the light for curing the adhesive can irradiate the adhesive layer through the polarization splitting film and the reflecting film of a dielectric multi-layer film structure, and this simplifies the fabrication process of the optical element. Further, the reflecting film of a dielectric multi-layer structure can be designed to have higher reflectance of a specific linear polarized light component. This further enhances the light utilization efficiency.
In a preferred embodiment, the light exit surface includes a first exit surface portion and a second exit surface portion. The first exit surface portion emits selected one of S-polarized light and P-polarized light which has passed through the polarization splitting film, while the second exit surface portion emits the other one of the S-polarized light and P-polarized light which has been reflected by the polarization splitting film and the reflecting film. The optical element further comprises a xcex/2 phase plate associated with selected one of the first and second exit surface portions. Accordingly, only one linearly polarized light will be emitted from the optical element.
According to another aspect, the present invention provides a projection display apparatus comprising: a light source for generating luminous flux including S-polarized light and P-polarized light; an optical element for receiving the luminous flux from the light source and emitting the luminous flux as selected one of S-polarized light and P-polarized light; modulating means for modulating the light exiting the optical element as a function of a given image signal; and a projection optical system for projecting the light modulated by the modulating means on a screen.
These and other objects, features, aspects, and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments with the accompanying drawings.